Enhancement of photovoltaic characteristics of nanocrystalline 2,3-naphthalocyanine thin film-based organic devices

In this work, nanocrystalline thin films of 2,3-naphthalocyanine (NPC) were successfully deposited by a thermal evaporation technique at room temperature under high vacuum (similar to 10 (4) Pa). The crystal structure and surface morphology were measured using X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. A preferred orientation along the (0 0 1) direction was observed in all the studied films and the average crystallite size was calculated. Scanning electron miscroscopy (SEM) images of NPC films at different thermal treatment indicated significant changes on surface level patterns and gave clear evidence of agglomeration of nanocrystalline structures. The molecular structural properties of the thin films were characterized using Fourier transform infrared spectroscopy (FTIR), which revealed the stability of the chemical bonds of the compound under thermal treatment. The dark electrical conductivity of the films at various heat treatment stages showed that NPC films have a better conductivity than that of its earlier reported naphthalocyanine films and the activation energy was found to decrease with annealing temperature. The absorption edge shifted to the lower energy as a consequence of the thermal annealing of the film and the fundamental absorption edges correspond to a direct energy gap. The temperature coefficient of the onset and optical band gaps for the film was calculated to be -4.4 x 10(-4) and -1.76 x 10(-3) eV/K, respectively. The effect of thermal annealing on the photovoltaic properties of Al/NPC/ITO devices was also considered. The as-deposited device showed maximum power conversion efficiency about 0.70% under illumination of 100 mW/cm(2), whereas 2.65% power conversion efficiency was achieved after annealing the samples at 500 K for 1 h. (C) 2012 Elsevier B. V. All rights reserved.